The present invention relates to sampling of aggregate media in liquid filtration systems and more particularly to apparatus for collecting a vertically continuous sample of media above an underdrain.
Municipal water treatment systems commonly use aggregate filtering systems, commonly referred to as multi-media filters, to remove particulates from water. These filters include multiple layers of aggregate materials in order to efficiently filter water. The bottom of such filters has an underdrain which may be formed of gravel or more modern man made materials, including plastics, rubber, stainless steel, etc. Typical filters have a sand layer immediately above and supported by the underdrain and a layer of anthracite coal particles above and supported by the sand layer. In typical designs, the sand layer is twelve inches thick and the anthracite layer may be eighteen to twenty-four inches thick. More modern regulations require the thicker layer of anthracite. Other filter layers of, for example, greensand or activated carbon may be required depending on the source of water, e.g. well water may contain hydrogen sulfide which can be removed with activated carbon.
The same type of liquid filtration systems may be used for removing particulates from wastewater or process water or other liquids in industrial operations.
In operation, the water or water based fluid to be cleaned flows by gravity down through the aggregate layers of the filter which trap particulates so that clear water flows into the underdrain. After a period of time, the aggregate bed collects enough particulate matter that its flow rate decreases and it must be cleaned. Cleaning is done by backwashing, i.e. flowing clean water into the underdrain and up through the aggregate bed. The upward flow fluidizes the aggregate and flushes the collected particulate matter out of the filter.
In order for an aggregate filter to operate efficiently, the aggregate layers must have certain minimum thicknesses in all areas of the filter. If the backwashing process works properly, the sand, anthracite and others layers naturally arrange themselves in the desired layered arrangement based on particle sizes and specific gravities according to Stokes law. However, the process sometimes does not work for various reasons. One common problem is the formation of mud balls in the aggregate bed. Mud balls are accumulations of coagulants and other deleterious materials that agglomerate in filter media if improper operation of the filter has occurred. Mud balls may have sizes and specific gravities such that they are not removed by backwashing. In other cases, they may simply stick to filter aggregate. In some cases, the underdrain malfunctions and does not uniformly flow water up through the aggregate bed during backwashing. In such cases the bed layers may become unevenly distributed to the extent that the filter does not efficiently remove particulate matter.
During backwashing and during normal operations, small amounts of the filter aggregate materials are commonly lost. If enough of the material, e.g. anthracite, is lost, the aggregate layers will no longer have the minimum required thickness and the filter will not operate as it should.
Filter efficiency is easily measured in terms of the turbidity of the filtered water. If too much particulate matter passes through the filter, the water will not be clear, i.e. it will be turbid. Optical test equipment can measure turbidity of the filtered water on a continuous basis. When the detected turbidity exceeds regulations, it means that the filter has failed and immediate action should be taken to correct the problem.
It is very desirable to monitor the condition of the aggregate layers in water filters so that corrective action can be taken before filter efficiency degrades to an unacceptable point. This is especially true during periods of low water consumption, e.g. winter months, during which times an inefficient filter may provide acceptable filtering due to low flow rate. If the flow rate is increased, e.g. during summer months, the filter may fail to meet turbidity requirements.
Despite the desirability of preventive maintenance monitoring of such filter systems, it is not commonly done due the difficulty of checking the aggregate beds. One common method for checking the condition of water filter aggregate beds requires draining of the filter, insertion of a transparent box into the aggregate bed and manual shoveling out of the materials for measuring layer thicknesses and taking samples for testing. Due to the down time and large labor requirement, this is not normally done until the filtered water fails to meet requirements, i.e. when a failure has occurred.
Attempts have been made to use metal seed samplers such as the grain probes sold by Seedburo Equipment Company of Chicago Ill. to sample aggregate materials used in water filters. During construction of new filtration systems such seed samplers have successfully been used to sample dry anthracite and granular activated carbon from semi-bulk containers. The same seed samplers were not found to be suitable for sampling dry silica gravel, silica sand, high-density gravel and high-density sand in such containers. These hard granular materials cause binding both on opening and closing of the seed sampler.
The seed samplers have a sharp heavy metal point designed to penetrate bulk seed. Such points could easily damage underdrains, especially those made of rubber or plastic. Metal sampling devices also represent a shock hazard in many locations, since the sampling device must be relatively long to be used in typical water filtration systems and must be raised overhead when being inserted into or removed from the filters. The overall length of such seed samplers make them difficult to transport from one location to another. Available seed samplers are of relatively small diameter and have a large number of small openings for collecting seed samples. This opening arrangement interferes with collecting a vertically continuous sample of filter aggregate and interferes with observation, e.g. of transition zones, and measurement of the aggregate layers.